Thermal ink jet printing has been described in many recent technical articles, such as an article by Kuhn & Myers in Scientific American, 1985, at pages 162 through 178, and also in an article by J. B. Angell et al. also in Scientific American April 1983 at pages 44 through 55, both incorporated herein by reference.
In the art of multitone ink jet printing, one approach to printing a dot with one of eight grey scale levels is to employ a single ink jet drop generator and fire it from one (1) to seven (7) times at a given pixel in order to provide the pixel from one to seven levels of ink drop volume. However, this approach suffers at least two distinct disadvantages when used in a thermal ink-jet printer. The first of these disadvantages is that the substantial repeated use of single drop generator and its associated heater resistor increases the wear and failure rate (decreased lifetime) of the thermal ink jet print head. As used herein, the term "wear" is defined as the accumulation of drop ejection cycles in a drop generator with finite lifetimes. Such lifetimes are typically measured in tens of millions of cycles.
Secondly, when ink is ejected in a drop sequence from a single drop generator, there is a certain recovery time related to the bubble collapse associated with each ink drop ejection from the drop generator. This recovery time obviously imposes a limitation on the maximum achievable rate at which pixels are printed using this method of thermal ink jet printing.
Another approach to multitone ink jet printing involves the use of multiple ink jet drop generators and firing these drop generators simultaneously in different numbers to achieve different corresponding ink jet drop volumes. To some extent, the use of these multiple drop generators as contrasted to a single drop generator can increase the lifetime of the thermal ink jet printer. One such approach is disclosed for example by T. Kawanabe in U.S. Pat. No. 4,353,079 issued Oct. 5, 1982. However, the thermal ink jet recorder apparatus of the Kawanabe patent identified above is possessed with certain other disadvantages related to the requirement for simultaneous firing of the multiple drop generators therein.
In particular, since these drop generators of the prior art are simultaneously fired at a single location, the nozzles must be critically aligned with respect to each other so that the ink drops will properly register within the pixel on the recording medium (paper). Furthermore, this alignment is predicated upon a particular spacing between the nozzles and paper, and maintaining this distance is critical to achieving a simultaneous combination of these drop volumes on the pixel. In addition, since each drop generator in the Kawanabe recorder of U.S. Pat. No. 4,353,079 produces only one unit volume of ink, then anywhere up to seven drop generators must be fired simultaneously to achieve the variation of one to seven levels on the grey scale. This requirement significantly increases the complexity,cost and unreliability of printhead design, and it also increases the total drop generator-use time for the print head and, again, imposes a limitation on the useful print head lifetime.